Method for charging plastic powder electrostatically by friction only

ABSTRACT

A method of coating particles with plastic material powder, such as epoxide resin powder without the use of an external electrical field comprises the steps of mixing the powder in a stream of pressurized air, accelerating the air and powder stream to supersonic velocity through a venturi device, electrostatically charging the plastic material powder by friction against a polytetrafluoro ethylene surface. The front portion of the nozzle discharging the material is grounded by a metallic sleeve.

United States Patent [191 Schaad [451 Sept. 2, 1975 METHOD FOR CHARGING PLASTIC POWDER ELECTROSTATICALLY BY FRICTION ONLY [76] Inventor: Hans J. Schaad, Erlenbachstrasse 37, 46 Dortmund-Schuren, Germany [22] Filed: Jan 22, 1973 21 Appl, No.: 325,231

[30] Foreign Application Priority Data Jan. 25, 1972 Germany 2203351 [52] US. Cl. 427/25; 427/27; 239/15; 239/602; 239/DIG. 19; 317/3 [51] Int. Cl. BOSB 5/02; B44D l/O94 [58] Field of Search 117/17; 317/3; 239/15, 239/602, DIG.19; 427/25, 27

[56] References Cited UNITED STATES PATENTS 2,477,947 8/1949 Yadoff 317/3 2,574,284 11/1951 Prochazka.

2,761,997 9/1956 Forest ct al 117/17 3,339,841 9/1967 Beach 1 i 239/15 3,434,859 3/1969 Benjamin.... 117/17 3,507,686 4/1970 Hagenbach..... l17/17.5 3,521,815 7/1970 Szasz 239/15 3,533,835 10/1970 Hagenbach ct a1. ll7/17.5 3,597,668 8/1971 Yoshimine 317/3 3,630,442 12/1971 Probst 239/15 Primary Examiner-Michael Sofocleous Attorney, Agent, or" FirmFitch, Even, Tabin & Luedeka [5 7] ABSTRACT A method of coating particles with plastic material powder, such as epoxide resin powder without the use of an external electrical field comprises the steps of mixing the powder in a stream of pressurized air, accelerating the air and powder stream to supersonic velocity through a venturi device, electrostatically charging the plastic material powder by friction against a polytetrafluoro ethylene surface. The front portion of the nozzle discharging the material is grounded by a metallic sleeve.

3 Claims, 3 Drawing Figures PATENTED SEP 2 975 sum 1 o 3 Z ll |lllallililllvlllillallrltf METHOD FOR CHARGING PLASTIC POWDER ELECTROSTATICALLY BY FRICTION ONLY The present invention relates to a method of coating articles with plastic material powder. expecially epoxide resin powder, in an electrostatic field, and an apparatus for carrying out such method.

It is well known to spray plastic material powder onto articles by means of accelerated air streams, whereby the particles of the plastic material powder are subjected to an electrostatic charge by an electrical field, so that they settle on the grounded or oppositely charged article because of the electrical attraction. Upon baking, the layer of plastic material powder provides a surface coating being superior to conventional lacquer coatings.

However, it is of disadvantage in the known powder coating methods and apparatuses that they require an electrostatic voltage source which, because of the high voltage used (between 30 and 150 kilovolts), necessitates expensive safety measures.

It is the object of the invention to provide a method and an apparatus for the powder coating of articles which do no longer show the abovementioned disadvantages and which, in particular, are more safe in operation, which render possible an ideal distribution of polarization and ionization which, otherwise, is disturbed by foreign charge, which are capable of operating without any additional voltage source for an electrostatic field, and wherein the apparatus is simple, easy and economical to manufacture and operate, and wherein it is possible to adjust the thickness of the coating by adjustment of the flow velocity or of the quantity of air, respectively.

According to the invention, this object is solved in that the plastic material powder is electrostatically charged by friction against polytetrafluoro ethylene (PTFE), and then sprayed onto the article to be coated.

Furthermore, the object according to the invention is solved in that the plastic material powder, after the selfcharging by mutual friction between the plastic material particles and/or with the nozzle walls, is further passed through highly insulating materials only which, preferably, have a very high resistivity of about from to 10 ohms per centimeter.

In a manner being particularly surprising to the expert, it has shown that the good coating according to the invention can be obtained not only by using polytetrafluoro ethylene, but that, further, polytrifluoro monochloroethylene and other fluoro compounds and solid, highly insulating materials, such as amber, mica and quartz glass, may be employed.

Prior condition for the usability is, on the one hand, a high insulating effect which substantially exceeds the insulating effect of e.g. standard glass and the like, and, on the other hand, a surface consistency which safely prevents the plastic material powder from adhering thereto. Additionally, the material must be chemically inert towards the plastic material powder, too.

In the following. an exemplary embodiment of the invention is explained in greater detail by referred to the accompanying three drawings.

FIG. 1 schematically shows in sectional view an apparatus for discharging electrostatically charged plastic material powder, whereby the powder reservoir is attached directly to the jet pipe.

FIG. 2a pistol for the powder coating of articles according to FIG. 1 and FIG. 3a pistol according to FIG. 2 but without having the powder supply attached to the pistol.

The powder spraying apparatus according to the invention comprises primarily a jet pipe I and a mixing section 2 positioned upstream thereof, both components being formed of polytetrafluoro ethylene. The jet pipe 1 per se may consist of a nozzle portion Ic having a subsequent expansion portion lb which is followed by the actual spray pipe la. The spray pipe Ia may merges with a diffusor 7 wherein the velocity of the outflowing powder/air mixture is lowered, and includes a perforated plate 6 which is likewise formed of polytetrafluoro ethylene and which provides a uniform flow distribution.

The powder enters the mixing portion 2 which preferably is a cylindrical chamber, via a passage 8. The passage 8 may be either directly surrounded by a powder reservoir 9, (FIGv 2), or the powder may be fed thereto over a certain distance (FIG. 3).

The rear wall of the mixing or distributor section 2 includes an opening into which a nozzle housing 4 opens. The nozzle housing which likewise consists of polytetrafluoro ethylene, has a nozzle needle passing coaxially therethrough, whereby the quantity of the entering air may be controlled by the position of this needle. Of course, it is also possible to introduce the air via the nozzle housing 4, whereby the quantity of air may be adjusted at some distance, e.g. in the pressurized air feed conduit. In such case, the nozzle needle may be omitted.

The apparatus according to the invention may be formed either as a hand spray gun by being provided with a handle 11 or it may be positioned with some distance from the powder reservoir either stationarily or so as to be movable (FIG. 3).

It is importaant for the generation ofa very high electrostatic field of e.g. up to 200 kilovolts that the whole quantity of powder within the accelerated air flow may contact the polytetrafluoro ethylene only, so that, on the one hand, there are produced ionized and polarized powder particles, and, on the other hand, the resulting charges are not dissipated afterwards.

In a manner being surprising to the expert, the selfcharging of the powder particles renders possible both a polarization and an ionization of the powder particles in ideal combination, so that, when selecting high flow velocities, high thicknesses of the coating in comparison with conventional coating methods may be obtained.

By adjustment of the flow velocity, the deposition of the powder particles may be controlled, so that smaller layer thicknesses may be adjusted, too.

By forming the nozzle with a minimum crosssectional area and a subsequent expansion zone within the extension of a continuous curvature, it is possible to produce supersonic flow velocities.

Surprisingly, the use of polytetrafluoro ethylene in combination with the flow velocity does not only result in that a voltage source becomes superfluous, but also, as mentioned above, in that a superior coating is obtained.

The charging and, thus, the coating depend on the temperature and on the humidity of the pressurized air, Therefore, it is advisable to provide for a pre-drying of the air when operating at high humidity. However, it has been found that an adequate coating by means of the method according to the invention can be obtained even at very high humidities (of above 90 "/1 Epoxide resin is particularly useful for the processing in accordance with the present method. It has been housing. In case of the pistol according to FIG. 3, where coating powder is supplied via tube 8, the control of the powder supply is effected by switch 16 as well.

found. however. that polyester. polyamide and other Referring now to the following table. a number of explastic material powders can be processed with good oriment were performed with apparatus constructed re l o by ing h me hod according to the inin accordance with the drawings using several different vcntion. materials or a combination of materials for the Venturi Surprisingly, the electrostatic properties of polytetranozzle 1 Also, several types of powder including epfluoro ethylene are superior to any other plastic materil0 oxide powders were used with the apparatus of the varials. Enclosed is a Table showing the surprising effect ous materials in attemping to determine the best comobtained in the processing of various types of plastic bination of materials and powders for use with the illusmaterials when using jet pipes and mixing ham r trated apparatus. It was discovered that the use of polymade Of polytetrafluo o hyle e Evidcmlyt A o g tetrafluoro ethylene, particularly when using cpoxide can still be obtained when using po yethylene; ho powders produced the best results. The coating formaever, the use of polytetrafluoro ethylene results in a imtion on the article sprayed was also analyzed. proved effect which is absolutely unexpected by the ex- M ifi ll an electrostatic measuring instrw pert and whi h. in i Of the p i lmOWIedgC ment was used to measure the charge of the powder HbOuI the effect of plastic parts in Powder p y guns, particles in micron amperes. When teflon is used for could not be foreseen, which fact is also demonstrated th atu the average charge was at least one miby the submitted compilation. cron amp and for the epoxide powders, the charge was In order to obtain the effect according to the inveneven 1.5 to 2.5 micron amps as shown in the Table. tion, it is essential that all portions of the mixed flow of R f i m articularl to the Table, it will be driving air and powder are in contact with polytetraeen that first Column is labeled Teflon has a first fluoro ethylene surfaces only, and that a sufficiently subcolumn with numerals showing the micron amperes high flow velocity within the jet pipe is produced by (uA) measured by the electrostatic measuring instruusing driving air of high pressure. ment for the various powders. Thus, the first five epox- Apparently, sonic flow velocity and supersonic flow ide powders measured at least one micron amp charge. velocity may be adjusted with the apparatus according In contrast thereto, when using such materials as PVC to the invention formed in the manner of a Laval nozor polycarbonate or polypropylene for the Venturi tube zle. already by using driving air having a pressure of the average charge of the powder was less than 1 and more than 2 atmospheres. in many cases zero.

Advahmgeously at least the from Portion of the j The coating formations were also analyzed and rated P P l 15 Surrounded by a grounded mctalhc Sleeve 3 during these experiments. A very good coating forma- Order to Prcvcht annoyance of the Operators by Spark tion is indicated in the Table by a numeral 1 in the secgchcmhoh the Surface of the J P P Furthermore, 0nd sub-column under the letter S which refers to coatthis metallic sleeve 3 prov1des a ccrtam improvement i f ti Th the fir t fi e e oxide wder had of the effect according to the inventio a very good coating formation only when Teflon was If spreading device I2 is m unted ithin th nOZZle used for the Venturi nozzle. Several other powders had 14 of jet pipe 1 e.g. by means Of ri s 13, this spr i g a good coating formation as indicated by the numeral device as well as the ribs and the nozzle 14 should con- 2 when u i Teflon. in contrast, the PVC, polycarbonsist of polytetrafluoro ethylene. According to FIGS. 2 ate and polypropylene coatings generally were labeled and 3jet tube 1 may be covered by an outer protective 3s, 4s, and 5s, with 3 being only usable, 4 being sleeve 15 made of PVC or the like. According to FIGS. 5 not usable and 5 indicating no adhesion at all. 2 and 3 switch 16 within the handle ii via a cable 17 by Thus, it is readily apparent that surprising and superior means not shown controls supply of pressurized air results were obtained by using Teflon and epoxide powflowing to the pistol through flexible tube 19 and via ders as set forth in the first upper left hand portion of removable nozzle 18, being screwed into the pistol the Table.

TABLE Coating Formation (S) Current (,u A) and Coating formation (S) l very good with Venturi nonles made of various 2 good materials and with the use of different 3 still usable powders 4 not more usable 5 no adhesion Teflon Glass- (iraph- PVC Hard Poly- Poly Poly- Poly- Alu- TYPE OF Teflon ite- Rubber ethcarproureminum POWDER Teflon ylene hon-ate pylene thane Metal #A S uA S p.A 5 [.LA S ;1.A 5 [LA S tA S [-LA A S p.A S

Epoxide +1.5 1 +1 1 +0.5 2 4 3 (1.5 2 0 4 0 4 (1 4 0 5 Standard Epoxide +2 1 +1.5 2 0 3 o 4 0 3 o 3 0 3 0 3 o 3 0 5 Graphite Epoxide +1 1 +1 2 +1 2 o 4 o 4 0 3 o 4 0 5 0 3 0 5 Black Epoxide +1 1 +1 2 +0.5 3 0 3 o 4 0.5 2 o 4 o 5 0 o 5 Metal Epoxide +2.5 1 +0.5 2 +1.5 1 +0.5 2 +0.5 2 +1 1 0 3 0 3 o 3 (1 5 White Polyester Thermoplast +0.5 4 +l. 2 +l.5 2 5 0 S 0 U 5 O 5 t) S O 5 TABLE Continued Coating Formation (8) Current A) and Coating formation (S) l very good with Venturi nonles made of various 2 good materials and with the use of different 3 still usable powders 4 not more usable 5 no adhesion Teflon Glass- (iraph- PVC Hard Poly- Poly- Poly- Poly Alu- TYPE OF Teflon ite- Rubber eth ear proureminum POWDER Teflon vlene honate pylene thane Metal ;.LA S ;LA S ,uA S #A S 1A S ,uA S ,uA S 4A S #A S #A S Polyesten curable U 5 5 l .5 2 5 5 (l 5 5 5 5 (l 5 Polvester 3 4 0 470 3 +0.53 0 4 O 3 0 4 +0.52 Polyester +5 2 +4 2 +05 3 U 5 O 5 U 4 l) 5 (l 5 (l 5 l) 5 Polvamidc +1 2 0.5 3 +0.5 3 +1 2 +0.5 2 2 +1 2 +l 2 Z (l 5 (327.05 PVC +0.5 3 +05 4 +0.5 4 0 5 (l 5 "()5 2 5 (l 5 U 5 (l 5 Polyurethane +0.5 3 -H),5 3 +l 2 (l 5 (l 5 t) 5 5 t) 5 (l 5 (l 5 positive;

negative: 0 no indication 1 Claim: tion of the charges on said particles, and discharging l. A method of coating articles with an epoxide pow der within an electrostatic field without applying an external charge to the article comprising the steps of: mixing the epoxide powder in a stream of pressurized air within a mixing section of polytetrafluoro ethylene, accelerating the air and powder stream to supersonic velocity through a venturi device of polytetrafluoro ethylene, electrostatically charging the material powder solely by friction of the material flowing against the polytetrafluoro ethylene in said venturi device, passing the charged powder through a spray device having a surface of polytetrafluoro ethylene to prevent dissipaand spraying the clectrostatically charged epoxide powder from the spray device onto the article to be coated.

2. A method in accordance with claim 1 including the step of: grounding a nozzle from which the spray is discharged.

steam of pressurized air within the mixing section.

* l f =l 

1. A METHOD OF COATING ARTICLES WITH A EPOXIDE POWDER WITHIN AN ELECTROSTATIC FIELD WITHOUT APPLYING AN EXTERNAL CHARGE TO AN ARTICLE COMPRISING THE STEPS OF: MIXING THE EPOXIDE POWDER IN A STREAM OF PRESSURIZED AIR WITHIN A MIXING SECTION OF POLYTETRAFLUORO ETHYLENE, ACCELERATING THE AIR AND POWDER STREAM TO SUPERSONIC VELOCITY THROUGH A VENTURI DEVICE OF POLYTETRAFLUORO ETHYLENE, ELECTROSTATICALLY CHRAGING THE MATERIAL POWDER SOLELY BY FRICTION OF THE MATERIAL FLOWING AGAINST THE POLYTETRAFLUORO ETHYLENE IN SAID VENTURI DEVICE, PASSING THE CHARGED POWDER THROUGH A SPRAY DEVICE HAVING A SUFACE OF POLYTETRAFLUORO ETHYLENE TO PREVENT DISSIPATION OF THE CHARGES ON SAID PARTICLES, AND DISCHARGING AND SPRAYING THE ELECTROSTATICALLY CHARGED EPOXIDE POWDER FROM THE SPRAY DEVICE ONTO THE ARTICLE TO BE COATED.
 2. A method in accordance with claim 1 including the step of: grounding a nozzle from which the spray is discharged.
 3. A method in accordance with claim 1 including the step of providing the epoxide powder in a powder reservoir and feeding the epoxide powder for mixing in the steam of pressurized air within the mixing section. 